This invention relates to the field of sensing the rotational speed of a target when the target is concealed from a sensor by another component.
Currently electronic transmission controls rely on accurate information regarding the rotational speed of transmission components located within a case. The speed signals are used as input information to sophisticated powertrain control algorithms. The speed of most components in the case can be accessed directly by magnetic sensing devices, but occasionally such access can only be obtained upon relocating the target component adjacent a sensor. Frequently these relocations compromise the power flow arrangement in the transmission and add cost and complexity to the mechanical design, hydraulic actuation and electronic controls. Indirect access using surrogate speeds in combination with algorithmic corrective calculations, in place of the true target speed, sacrifices response time and accuracy.
When the target component has interposed between it and the speed sensor another component formed of ferrous metal, magnetic flux exchange between the sensing device and the surface profile of the target element will be attenuated. To avoid this difficulty it is preferable that the interposed component have low magnetic permeability while providing high structural strength.
U.S. Pat. No. 5,825,176 describes an apparatus in which the speed of a rotating inner member is represented by a signal produced by a speed sensor located adjacent the outer surface of an outer member, which covers at least partially the inner member. The outer member is formed with a pattern of angularly and axially spaced windows through its thickness. These windows provide intermittent direct access of a magnetic flux path from the sensor to the target component and interrupted direct access as each window rotates past the flow path.
In an alternate approach using non-magnetic material for the interposed element, a high cost magnetic ring is pressed onto the target component in order to provide sufficient magnetic signal penetration through the interposed outer component.